An apparatus for removing fine material to be screened from a liquid is known from German Patent No. DE 34 20 157 C1 corresponding to U.S. Pat. No. 4,634,524. The known apparatus includes a cylindrical sieve grate. The sieve grate with its axis is submerged in an inclined manner in the channel though which the liquid flows. The sieve grate is designed to be hydraulically opened at its inflow side and to be substantially hydraulically closed at its outflow side. The sieve grate includes a perforation of slits forming a separation surface at the inner side, while the liquid passes through the slits and remains in the channel. The cylindrical sieve grate is rotatingly driven in connection with a screw conveyor. The screw conveyor begins with a feeding hopper being coaxially arranged and supported in the region of the sieve grate in a stationary manner. The screw conveyor includes a housing and a conveying screw. A stationary detaching device designed as a brush roll or a spray rail is arranged at the outer side of the sieve grate and above the feeding hopper. The detaching device serves to detach the material to be raked and/or to be screened which adheres to the inner surface of the sieve grate. The material is detached from the outside. The screenings or rakings fall down in the feeding hopper which is the first element of the screw conveyor. The screenings or rakings are then upwardly conveyed to a discharge location located outside of the liquid. The cylindrical sieve at its inner side grate includes guiding plates being arranged in a spiral configuration. The guiding plates have the function of conveying pieces of rakings (especially bigger ones) in an upward direction during rotation of the sieve grate and of preventing them from falling back during rotation of the sieve grate. The openings of the perforation of the sieve grate may be designed to have a size from between a plurality of millimetres to approximately 10 mm. Smaller openings cannot be produced in an economic way. Consequently, the known apparatus cannot be used for treating sludge-like screenings.
A rotating filter is known from German Patent Application No. DE 24 26 024 A1 corresponding to British patent No. 1 469 310. The known rotating filter serves to clear liquids from mud in an automatic and continuous way, the liquids containing impurities. A rotatingly driven cylindrical sieve grate designed as a filtering drum is used. The filtering drum includes two disc-like faces between which a sieve is tightened. The two faces are maintained at a distance with respect to one another by bars. Preferably, the sieve is freely tightened to the disc-like faces by elastic supporting rings such that it may be easily replaced. However, the sieve may also be connected to the faces at its circumference by fixing brackets or by other devices. The apparatus is arranged in a way that its axis is located in a horizontal direction, and it includes a feeding hopper and a screw conveyor for discharging the screenings in an axial direction.
A sieve grate having a three-part design is known from U.S. Pat. No. 3,747,770 and U.S. Pat. No. 5,399,265. The known sieve grate at the inside includes a mesh sieve having small meshes. The mesh sieve contacts a grid having large openings. As the third part of the sieve grate, a drum is arranged at the outside. The drum provides for stability and resistance, and it is made of a respective number of transverse bars and circumferential elements. The mesh sieve is connected to the grid construction by a sintering process such that a majority of contact points between these two elements is realized.
A rotating filter for separating solid materials from a liquid is known from German Patent Application No. DE 24 02 871 corresponding to U.S. Pat. No. 3,936,378. The known rotating filter has an octangular box-like design over the outside of which a filtering net is tightened. The liquid containing the screenings flows through the filtering net from the inside towards the outside such that the screenings are deposited at the inner side of the filtering net. A detaching apparatus is arranged outside of the filtering net. The detaching apparatus operates with compressed air to eject the screenings into a feeding hopper of a conveyer screw and to discharge the screenings from the region of the sieve drum in an axial way.